Duct tape really was originally called duck tape. The name comes from canvas-backed tape upon which it was based when introduced in World War II. A once-common name for the type of canvas used in the tape was “cotton duck”.
— William W. Lawrence Sr. 7-31-17 Duck Tape
Still is, too, by the way. You’ll hear people say both. It’s a elision, dropping a vowel or consonant in pronouncing a word or words. It’s similar to the -d and -t eliding in “iced tea”, coming out as “ice tea”.